The present invention relates to occupancy sensors for controlling lights and other electric loads by detecting doppler shifts in transmitted ultrasonic sound.
An occupancy sensor will typically transmit ultrasonic sound waves via one or more transmitters which then reflect off of objects in the room and are detected by one or more receivers. A doppler shift of the reflected signals indicates the presence of movement within the room, and can be detected by noting frequency changes from the transmitted frequency. In the absence of a detected doppler shift, an electric load can be switched off, thereby conserving energy when no people are in the room. Some examples of such systems are shown in U.S. Pat. No. 3,781,859, U.S. Pat. No. 3,383,678, U.S. Pat. No. 3,721,972, U.S. Pat. No. 4,172,253 and U.S. Pat. No. 4,021,679.